The present invention relates to methods and apparatus for mixing streams of flowing materials and more particularly concerns such mixing without movement of parts other than the flowing material itself.
Static mixers, also termed interfacial surface generators, having a number of advantages over dynamic mixing devices and have been used for some time. Various types of these mixers have been developed, fundamentally employing apparatus for flowing materials and for repetitively combining, dividing and recombining streams of the materials many times. The number of combinations, divisions and recombinations depends upon the degree of mixing required or desired and certain characteristics of materials being mixed, particularly viscosity. Typical of static mixers presently known are those described in U.S. Pat. Nos. 3,051,453, 3,195,865, 3,239,197, 3,328,003, 3,643,927, 3,652,061 and 3,286,992. In general, such static mixers embody an elongated linear flow path in which a number of flow rotators or twisters are provided. Such rotators or twisters divide the flowing stream into two branch streams and effect a mixing of the material within each of the branch streams as it flows through the rotator. This mixing is effected by a configuration of the rotator that causes the material to flow through a rotating or twisting path, rotating about an axis substantially aligned with the direction of flow. By rotating material in each branch stream, radial forces and eddy currents are generated, and thus the material within the stream is mixed. From another point of view, the action of these rotators may be described as a cross-sectional distortion of alteration wherein the branch stream in an upstream end of the rotator has an elongated cross-section with major and minor axes oriented in one direction and, at a downstream end of the rotator, has a similar elongated cross-section but with its major and minor axes oriented in another direction. Thus, the rotator may be considered a device for altering the cross-section of the branch stream or for rotating the stream about its direction of flow.
More viscous materials are more difficult to mix and it is found that a large number of mixing elements or flow rotators are required to effect satisfactory mixing of materials with higher viscosity. Ten or more of such elements may be placed in series in a flow path for viscous materials. For mixing polyester resins such as urethanes, for example, it is common to employ a tube having 30 or more individual mixing or rotating elements mounted in series therein. A commonly employed static mixing device of the type shown in U.S. Pat. No. 3,286,992 is distributed by Kenics Corporation and is available in tubes having from 15 to 27 individual mixer elements positioned therein. For mixing more difficult materials or for obtaining a greater degree of mixing, one must employ more individual mixing or rotating elements, thus requiring a longer tube. The longer the mixing apparatus, the greater are problems of providing space for the mixer, and the greater the back pressure or resistance to flow. However, an even more significant problem arises in the use of the mixer for materials that cure or harden. Unless the device is used continuously or is cleaned immediately after each use, it will become clogged with material that hardens within the tube. This problem is recognized by the patentees Armeniadis et al in U.S. Pat. No. 3,286,992 who state that after a run of resin, for example, or other setting material, it may be advantagesous to discard the device rather than attempt to clean it. Obviously, according to the patentees either one or the other must be done as otherwise the resin will set within the tube and render it useless. Thus, the difficulty of employing such static mixers with viscous, settable material, is recognized but the suggested solution is unsatisfactory. In view of the exceedingly high cost of such devices disposability is not economical.
Other problems encountered with existing static mixers include the difficulty of securing the individual mixing elements in desired position and thus arrangements, such as those suggested in U.S. Pat. Nos. 3,652,061 and 3,827,676 have been suggested. These arrangements increase the complexity of manufacture and thus increase the cost.
Schippers et al U.S. Pat. No. 3,206,170 shows a mixer in which a number of pairs of mixer elements are adjoined, but axially directed channels are arranged so that flow division occurs only between pairs of elements and not between elements of a pair.
Accordingly, it is an object of the present invention to provide a static mixer that avoids or minimizes the above-mentioned problems.